The invention relates generally to an internal-gear machine such as an internal-gear pump.
In internal-gear machines such as a pump, an externally toothed pinion or pump impeller is mounted eccentrically with respect to the body of the pump, for actuating an internally toothed annular gear which is disposed rotatably in the casing of the pump. In pumping operation of such a pump, the fluid which is in the pressure chamber of the pump is displaced out of the gaps between the teeth of the meshing externally toothed pinion and the internally toothed annular gear, by virtue of the increasing engagement or meshing between the teeth of the pinion and the annular gear so that the fluid is conveyed to the outlet of the pump. By virtue of that displacement effect, the fluid is obliged to flow away axially and radially past the teeth of the pinion and the annular gear, which pass into the gaps between the teeth of the annular gear and the pinion respectively. The gap which is available for that flow of fluid between the meshing teeth is however already comparatively small at the beginning of meshing engagement between the teeth so that there is already a comparatively high level of flow resistance to that flow of fluid long before the meshing teeth come into face contact with each other. That flow resistance rises further when the gap available for the flow of fluid between the meshing teeth further decreases in size with further increasing meshing engagement of the teeth and finally falls to zero. That high level of flow resistance manifests itself as a loss of output from the pump. In an endeavour to reduce that power loss, one design of internal-gear pump provides that the annular gear thereof has radial openings which communicate the external periphery of the annular gear with the inward side thereof in the region of the gaps between the teeth, so that the fluid being delivered by the pump is passed to the pump outlet through the opening through the annular gear. The presence of the openings through the annular gear means that, when the gaps between the meshing teeth of the annular gear and the pinion decrease, the fluid can also leave the gaps between the teeth of the meshing components in the direction of the openings through the annular gear so that the phenomenon of fluid being squeezed between the meshing teeth of the gears does not occur, in particular just prior to the condition of full meshing engagement between the teeth. A further improvement along those lines is to be found in another design of internal-gear machine, as disclosed in U.S. Pat. No. 1,739,139, in which, in addition to the openings which communicate the external periphery of the annular gear with the gaps between the teeth thereof, there are further openings which go from the external periphery of the annular gear through the teeth thereof to the tips of the respective teeth and open precisely at the tips of the teeth. By virtue of that arrangement, any fluid which is present and squeezed between the tips of the teeth of the annular gear and the gaps between the teeth of the externally toothed pinion is also conveyed away to the pressure chamber of the pump in the same manner as described above. However the openings in that internal-gear pump can only be comparatively small by virtue of their positioning in the teeth of the annular gear because, if the openings were not small, the strength of the annular gear would be excessively impaired by the openings being of large size and necessarily adjoining each other in close succession. The small size of the openings means that they are therefore of a narrow flow cross-section and therefore in particular the openings which open at the tips of the teeth of the annular gear still afford very narrow flow passages which present a not inconsiderable degree of flow resistance to the fluid to be displaced therethrough. Therefore in that machine the reduction in the loss of power due to the high resistance to flow of the fluid displaced by operation of the machine is only within narrow limits.
In connection with an external-gear pump, in order to remove fluid which is present and squeezed between meshing teeth, radial openings are provided in one of the two intermeshing externally toothed gears, the openings being displaced into the flanks of the respective teeth so that they intersect both the addendum circle and the dedendum circle of the teeth and thus communicate the region of the tips of the teeth and the region of the roots of the teeth with the shaft mounting bore of the gear, as can be seen from German laid-open application (DE-OS) No. 18 02 984. Although that arrangement avoids fluid being trapped between the teeth of the meshing gears, and the resulting loss-generating increase in pressure at the end of the phenomenon of meshing engagement between the co-operating teeth, the part of the fluid being conveyed by the pump, which is carried away to the mounting bore of the gear, represents a not inconsiderable loss of delivery volume from the pump.